[Simh] PDP-11 -- MFPI, etc, with memory mgmt off

[Bob Supnik]

If the MMU is enabled, the instructions use PSW<prev_mode> to compute 
the source/destination operand physical address (if memory) or the stack 
pointer to use (if SP). If the MMU is off, PSW<prev_mode> only matters 
if the source/destination operand is SP.

/Bob

On 9/22/2019 12:00 PM, simh-request at trailing-edge.com wrote:
> Message: 4
> Date: Sun, 22 Sep 2019 09:01:42 +0200
> From: Johnny Billquist<bqt at softjar.se>
> To:simh at trailing-edge.com
> Subject: Re: [Simh] PDP-11 -- MFPI, etc, with memory mgmt off
> Message-ID:<158acaa4-22c1-bb9c-ecdf-3c2b5d273f70 at softjar.se>
> Content-Type: text/plain; charset=utf-8; format=flowed
>
> Well, technically, on a Unibus, even with the Unibus map, a DMA device
> still only sees an 18-bit address. Or am I confused? But if the map is
> enabled, then it will end up being able to access all 4M of memory anyhow.
>
> But your answer made me wonder. Are you saying that M[TF]P[DI] are not
> using the previous mode bits in the PSW (when MMU is enabled)? Oh, maybe
> you are saying that even if MMU is disabled, the selection of which R6
> to use is still happening, based on PSW.
>
>     Johnny
>